Aspects of the present invention are directed to thermal lag mitigation in gas turbines using inlet bleed heat (IBH) systems and compressor interstage bleeds.
Typically, in a gas turbine, including a compressor, a combustor and a turbine, start bleed or interstage bleed air is removed from the compressor to regulate compressor loads during start-up operations and dumped into the gas turbine exhaust downstream of the turbine. Also, an IBH system is employed at a late stage of the start-up operations to remove hot air from an aft portion of the compressor and to subsequently deliver the hot air to a filter housing upstream from the compressor for recirculation through the compressor.
Generally, the IBH system includes relatively large quantities of piping, valves, manifolds, etc., all of which are usually metallic. As such, when the IBH system is cold, a considerable amount of time may be required for the IBH system to heat-up once the IBH system is activated and, while the IBH system absorbs heat from the compressor flow, the inlet temperature of the compressor air is changing with time. For some units, once a cold IBH system has been activated, nearly 30 minutes may be required for the compressor inlet air to reach a steady temperature. This issue becomes more significant when the gas turbine is placed in an extremely cold environment. In these cases, the start-up schedule may have to be altered to account for the thermal lag and, thus, control complexity must be increased with turndown capability reduced. Indeed, it has been seen recently, that control strategies have been significantly hampered in having to account for thermal lags in the IBH system.
As an additional matter, it has been observed that there is a lack of mixing in the exhaust stream once the start bleed air is reintroduced into the exhaust stream. This lack of mixing results in temperature gradients in, e.g., the associated heat recovery steam generator (HRSG) that can be unfavorable. Also, the staging of the IBH system and the start bleeds tends to cause undesirable transient airflow shifts in the combustor.